Spitzer measured the dip in infrared light as the planets went behind their stars

Scientists have reported a breakthrough in their quest to detect light from planets outside our Solar System.

Two separate teams in the US used the Spitzer orbiting telescope to measure infrared wavelengths associated with two distant, giant planets.

The data confirmed to the scientists that the planets, designated HD 209458b and TrES-1, must be extremely hot.

The work is published in the current edition of Nature magazine and a future copy of Astrophysical Journal.

"It's fantastic," said Dr David Charbonneau, of the Harvard-Smithsonian Center for Astrophysics in Massachusetts and the lead author on the Astrophysical Journal paper.

"We've been hunting for this light for almost 10 years, ever since extrasolar planets were first discovered."

Subtraction trick

About 130 planets have now been found outside our Solar System. Their existence has been established only indirectly, however.

Most were discovered using the so-called "wobble" technique - a planet is detected by the gravitational tug it exerts on its parent star; it makes the star jig back and forth in the view of Earth telescopes.

These observations are made using visible-light instruments and reveal the mass and size of planets.

"Spitzer has provided us with a powerful new tool for learning about the temperatures, atmospheres and orbits of planets hundreds of light-years from Earth," said Dr Drake Deming, of Nasa's Goddard Space Flight Center, and the lead author on the Nature paper.

It involves a little trick. First, Spitzer is used to collect the total infrared light from both the star and its planet. Then, when the planet dips behind the star as part of its regular orbit, the astronomers measure the infrared light coming from just the star.

The second observation subtracted from the first gives the infrared light that belongs to just the planet.

Unlike Earth

The exoplanets get their warm infrared glow because they are orbiting extremely close to their parent stars. They heat up to temperatures of almost a thousand degrees Celsius, the researchers report.

These are bloated Jupiter-type planets, and are unlikely to be home to anything living.

But the long-term hope, as techniques improve, is to see the light from more Earth-like planets and detect within it clues to life in remote parts of our galaxy.

What we are really doing in studying planets orbiting other stars (is) to really see whether our own Solar System is fairly commonplace, or really unique

Dr David Charbonneau

Dr Alan Boss, from the Carnegie Institution of Washington, said: "1995 was the beginning of the era of extrasolar planet discovery.

"But 2005 will go down in the history books and text books as being the year in which we have our first strong evidence for actually detecting light from a planet in orbit around a Sun-like star.

"It's hard to believe how much progress we have made in the last 10 years."

Dr Charbonneau added: "What we are doing in studying planets orbiting other stars (is) to really see whether our own Solar System is fairly commonplace, or really unique. And it's really through studies like what we have presented here that allow us to compare the planets of other stars directly to the planets of our own Solar System."

HD 209458b, nicknamed Osiris, is orbiting a star 153 light-years from Earth in the constellation of Pegasus. It circles the star every three and a half days at a distance of about seven million km.

TrES-1 is even further away, 489 light-years distant in the constellation Lyra. It orbits its star every three days at a distance of just six million km.

Earth, by contrast takes 365 days to orbit the Sun at a distance of 149 million km.